Most of the eastern half of the country has between 750 and 1000 millimetres (mm) of rainfall in the year. Rainfall in the west generally averages between 1000 and 1250 mm. In many mountainous districts rainfall exceeds 2000mm per year. The wettest months, almost everywhere are December and January. April is the driest month generally but in many southern parts, June is the driest. Hail and snow contribute relatively little to the precipitation measured.
Rainfall amounts are collected by a standard gauge consisting of a funnel and a container. The 15 Meteorological Service stations measure these amounts every hour but most of the 750 or so rainfall stations around the country read their gauges once a day. A few gauges in remote mountain locations are only visited once a month.
* Driest year 1887-only 356.6mm of rain recorded at Glasnevin Dublin
* Longest absolute drought Limerick 3rd April to 10th May 1938
* Greatest monthly total 790.0mm Cummeragh Mtns October 1996
* Greatest annual total 3964.9mm Ballaghbeena Gap 1960
* Greatest hourly total 97mm Orra Beg, Antrim, August 1980
* Greatest daily total 243.5mm Cloore Lake Co. Kerry 18 September 1993
How Often Does it Rain?
The general impression is that it rains quite a lot of the time in Ireland but in fact two out of three hourly observations will not report any measurable rainfall. The average number of wet days (days with more than 1mm of rain) ranges from about 150 days a year along the east and south-east coasts, to about 225 days a year in parts of the west.
How Heavy is the Rain?
Unlike the rain in many other countries, especially in the tropics, average hourly rainfall amounts in Ireland are quite low, ranging from 1 to 2mm. Short-term rates can of course be much higher: for example, an hourly total of 10mm is not uncommon and totals of 15 to 20mm in an hour may be expected to occur once in 5 years. Hourly totals exceeding 25mm are rare in this country and when they do occur they are usually associated with heavy thunderstorms.